Abstract: A system and method for controlling the speed of a motor operating in an extremely low speed range and having a rotary pulse encoder which outputs a pulse whenever a rotation axle of the motor has rotated through a predetermined angle. The extremely low speed range is defined such that its pulse interval of the output pulses is longer than a speed control period of the system are disclosed, load torque estimated value observer is provided. The least order disturbance observer includes a first calculating block which calculates an estimated value of the motor speed n.sub.M ' (j) of a motor model on the basis of a torque command and a load torque estimated value: a second calculating block which calculates an average value of the motor speed at each pulse interval n.sub.M ' (j).